The Future of Wireless LANs

Predicting the future of any technology is always challenging, but in this chapter, you learn about some current and upcoming technologies that might change the face of WLANs. Several of these technologies are not actually suitable for wireless LANs (WLANs) but might well provide complementary solutions.

The first technology we consider is Bluetooth, which is actually a current technology designed for short-range wireless, essentially as a cable replacement over distances typically limited to less than 10 feet. Bluetooth is often referred to as a personal-area network (PAN). Then, we discuss ultra wide band (UWB), an extension of Bluetooth with much higher data rates that uses short duration and low power pulses. On the other end of the spectrum, free space optics (FSO) have long been studied as a point-to-point wireless technology that leverages fiber-optic transponders to transmit gigabits of data without using optical fiber. Finally, 100 Mbps WLANs are in the early development stages to extend 802.11 and will most likely be the next-generation technology adopted for the applications described in this text.

As indicated earlier, some of these technologies might serve complementary markets and solutions to WLANs, some might supplant 802.11 WLANs, and some might die out.

Bluetooth

Of all the technologies considered in this chapter, Bluetooth is likely the most advanced technology from the development perspective. Additionally, acceptance of Bluetooth devices and solutions creating PANs has grown since the technology was made available to consumers several years ago. As stated earlier, the design intent of Bluetooth is to replace cables that would connect devices that are within close proximity, less than 10 meters (m), such as a computer and keyboard or possibly even on a person with a cell phone and microphone. Figure 9-1 shows an example that uses Bluetooth wireless links instead of messy and tangling cables to connect peripherals to the desktop PC.

Figure 9-1. Cable Replacement by Bluetooth

Bluetooth devices operate in the same 2.4 GHz Industrial, Scientific, and Medical (ISM) band as 802.11 and 802.11b WLAN devices, and many of the same Federal Communications Commission (FCC) part 15 rules govern their use and emissions. These rules require that they don't cause harmful interference to licensed users and state that they have no protection from interference from other users, licensed or unlicensed. This point is important, because in the future, they will likely be a common source of interference to WLANs and vice versa as laptop manufacturers begin to embed both devices into their product offerings. Similar to WLANs, they are also subject to interference from microwave ovens and cordless telephones.

Each device on a Bluetooth network (or piconetwork, a small, self-contained network) is either a master or a slave. The master initiates the wireless links, and the slaves respond to the master. In general, any Bluetooth device can be a master or a slave and can change roles or even assume both roles in different networks. A Bluetooth multipoint network can have up to seven active slaves per master. All the slaves communicate only to the master, so any communication between slaves must pass through the master. Scatternets are created when a device is a slave in more than one piconet or when it is a master in one or a slave in the other.

Most Bluetooth devices provide an effective isotropic radiated power (EIRP) of 0 decibels per milliwatt (dBm), although the specification does define three classes of Bluetooth devices:

Class 1 transmitters can provide up to 20 dBm (100 milliwatts [mW]) but must employ transmit power control so that they only employ the minimum power necessary for reliable communication.
Class 2 transmitters have a maximum transmit power of 4 dBm (2.5 mW).
Class 3 transmitters provide 0 dBm (1 mW).

Because Bluetooth was designed as a cable replacement, often for battery-powered devices, Class 1 Bluetooth transmitters are not common. The Bluetooth modulation scheme employed is Gaussian frequency shift keying (GFSK), just like frequency hopped spread spectrum (FHSS) WLANs, with a symbol rate of 1 Msps resulting in a base data rate of 1 Mbps.

Similar to older 802.11 WLANs, Bluetooth is a time division duplex (TDD) access mechanism using FHSS. The 2.4 GHz ISM band is divided into 79 1 MHz channels by one Bluetooth scheme, and each piconet hops through the channels in a pseudo-random manner. Bluetooth devices transmit each packet on a new hop channel. For a point-to-point or single slave piconet, 625 microsecond slots, each on a new channel, are created and numbered, with the master transmitting in even slots and the slave in odd slots. Each slot allows the transmission of 366 bits. With multipoint piconetworks, once again the master transmits in the even slots, but any given slave can only transmit if the packet in the previous slot was addressed to it. All slaves receive broadcast packets, but none can transmit in the timeslot following a broadcast packet. Because it breaks up the data to be sent into 366-bit packets, the protocol overhead can be quite high, so the Bluetooth specification allows the transmission of three-slot or five-slot multislot packets. They are transmitted on the same channel, and when the transmission finishes, the next transmission occurs on the channel that would have been used had the multislot packet not been present. In other words, some of the channels in the hop sequence are skipped. Either masters or slaves can use multislot transmission. Figure 9-2 shows a sample transmission sequence on a multipoint network with two slaves, with the master utilizing a multislot packet to one slave.

Figure 9-2. Sample Bluetooth Transmission Sequence

Bluetooth provides two different types of physical links:

Asynchronous connectionless links (ACLs) are most often used for data communication where data integrity is often a much higher priority than latency. Packet retransmission corrects error packets.
Synchronous connection-oriented (SCO) links create a circuit-switched, scheduled, low-latency point-to-point link between a master and a slave with no packet retransmission.

Each Bluetooth device has a unique 48-bit Bluetooth device address. Active slaves are provided with a 3-bit active member address by the master, whereas inactive or parked slaves are given an 8-bit parked member address. These parked slaves synchronize to the master's timing and hop sequence, and they listen for broadcast packets, where the master uses the parked member address to unpark them. The master also assigns them an access request address that specifies a special access window in which they can send an unpark request. As stated earlier, all Bluetooth devices can be either a master or a slave, and a master is nothing more than the device that initiates a piconet, whereas the slave is the device that enters the piconet at the request of the master. A master can initiate low power modes, sniff, hold, and parkto conserve energy, to allow more than seven slaves in a piconet, to provide the master with time to bring other slaves into the piconet, or to provide a means to be in multiple piconets, creating a scatternet.

Because scatternets are a bit different from anything that 802.11 WLANs provide, it is useful to examine them in more detail. The three most common uses for a scatternet are

To provide a mechanism for a device to enter an existing piconet by forming a scatternet with the master
To enable cross-piconet communication
To create a limitless store-and-forward network

Although these tools might appear to be rather useful, Bluetooth faces some challenges. For the devices themselves, they must maintain synchronization with two independent piconets. From a throughput perspective, the timing offsets between the two piconets reduce performance, and higher-layer protocols face challenges with routing and error recovery. With ACL traffic, a scatternet member can use the sniff, hold, and park modes to manage the two piconets, but with SCO traffic, each scatternet member must alternate between its two piconets. All this work can create such significant challenges that in the end, you might be better off having your device disconnect from one piconet before connecting to another one or installing two Bluetooth devices with a single host.

The Bluetooth Special Interest Group (SIG) that created the Bluetooth specification, and that manages the ongoing technical working groups in addition to other activities, actually defined several usage models for specific applications using devices from different vendors. Usage models include, but are not limited to, the following:

The three-in-one phone provides cellular and cordless phone operation in addition to walkie-talkie functionality.
The ultimate headset provides an audio interface to other devices such as telephones, computers, and stereo systems.
The Internet bridge allows a cellular phone to provide a bridge between Internet access via the cellular network and a computer with a Bluetooth interface.
The object push and file transfer usage models permit the basic transfer of information between enabled devices.

Mapping the appropriate profiles, which are the basic Bluetooth building blocks, creates these usage models. These profiles

Allow developers to reduce the many options that Bluetooth provides to only those that are required for the necessary function
Provide procedures for a function to be taken from a base set of standards
Provide a common user experience across devices from different manufacturers

In summary, Bluetooth by design solves a different problem than 802.11 addresses, that of cable replacement. As such, it is characterized by a lower transmission rate, a shorter range, lower power consumption, and lower cost in general. Because both Bluetooth and 802.11 WLANs operate in the same frequency band and are potential sources of interference to each other, it will be interesting to see how laptop manufacturers solve the problem of collocated Bluetooth and 802.11 devices.

UWB

UWB is a new technique for which the FCC has defined preliminary guidelines for using extremely wide relative bandwidth signals generated via short, low power pulses and that could allow high-bandwidth, interference-resilient communication. The FCC defines UWB as a signal that has a fractional bandwidth, the ratio of the signal bandwidth to the carrier frequency, of greater than 25 percent. The FCC guidelines allow you to transmit UWB signals across a wide breadth of spectrum that is already occupied by many other incumbent technologies which are relatively narrowband relative to a UWB signal. This arrangement is allowed under the principle that the emission limits are so low, even with the large numbers of transmitters, there is no perceptible impact upon existing technologies and systems. UWB systems use the very wide bandwidth to separate out the narrowband interference from existing systems. At press time, no standard exists for the pulses, their frequency, or the modulation technique, but nonetheless the technology holds great promise.

The FCC Report and Order that specifies new rulings creates several classes of UWB devices, each with its own set of emissions limits:

Low-frequency imaging systems consisting of ground penetrating radars (GPR)
High-frequency imaging systems consisting of GPRs, wall imaging, and medical imaging
Mid-frequency imaging systems for through-wall imaging and surveillance systems
Indoor communication and measurement systems
Outdoor handheld communication and measurement systems
Vehicular radar systems for collision avoidance, improved airbag activation, and suspension systems

The limits for the classifications, with the exception of vehicular radar systems, are summarized in Table 9-1.

Table 9-1. UWB Emissions Limits
Classification	Part 15 Frequency Band	Part 15 Emissions Limits (dBm/MHz)
		< .960 GHz	.9601.61 GHz	1.611.99 GHz	1.993.1 GHz	3.110.6 GHz	> 10.6 GHz
Low-frequency imaging	< 960 MHz	-41.25	-65.3	-53.3	-51.3	-51.3	-51.3
High-frequency imaging	3.110.6 GHz	-41.25	-65.3	-53.3	-51.3	-41.3	-51.3
Mid-frequency imaging	1.9910.6 GHz	-41.3	-53.3	-51.3	-41.3	-41.3	-51.3
Indoor	3.110.6 GHz	-41.3	-75.3	-53.3	-51.3	-41.3	-51.3
Outdoor	3.110.6 GHz	-41.3	-75.3	-63.3	-61.3	-41.3	-61.3

Vehicular radar systems have a pass band that extends from 22 to 29 GHz. Table 9-2 summarizes the vehicular classification emissions limits.

Table 9-2. UWB Emissions Limits for Vehicular Radar Systems
Classification	Part 15 Frequency Band	Part 15 Emissions Limits (dBm/MHz)
		< .960 GHz	.9601.61 GHz	1.6122 GHz	2229 GHz	2931 GHz	> 31 GHz
Vehicular	2229 GHz	-41.3	-75.3	-61.3	-41.3	-51.3	-61.3

As you can see, these emissions levels are quite low. In fact, they are at or below the spurious emissions limits for all intentional radiators and at or below the unintentional emitter limits. The emissions limits for ISM devices are a good 40 dB higher than what is called out by the FCC for UWB, so UWB signals should just appear as random noise to most receivers. From the perspective of interference to UWB from other radios, the large processing gain that the very high fractional bandwidth enables should remove the narrower band interference. With regards to multipath, the high fractional bandwidth also allows for a large pulse-separation period, relative to the pulse duration, so RAKE receivers should be able to constructively use multipath energy.

NOTE

In addition to producing emissions in the desired channel and band of operation, all radiators also generate unintentional or spurious emissions at other frequencies. In fact, many electronic devices that are not communication devices, such as microwaves, produce spurious emissions. The strength of these emissions is very tightly restricted by spurious emissions limits.

NOTE

A RAKE receiver takes the multiple copies of a transmitted signal that are generated by the unique propagation paths which create multipath and combines them to form a stronger composite signal than could be generated by any of the individual copies. When the duration of the pulses of a Bluetooth waveform are very short relative to the separation in time between pulses, the RAKE receiver is better able to separate out the copies.

The major challenges that UWB faces follow:

UWB calls for the design of RF devices of extremely wide RF bandwidth, devices that do not exist today. It is the nature of the technology that it will always be operating in the presence of interference at power levels much higher than the desired signal.
The bandwidth of the signal requires faster processing than can be done digitally today.
As with the RF challenge, there will be a similar challenge to design antennas with the desired bandwidth.
UWB is only an FCC initiative, so a major global standardization effort is necessary.

UWB is obviously well in front of the bleeding edge of technology and it will need to overcome many challenges. Over time, however, if the fundamental principles are correct, it could provide a wireless revolution similar to what 802.11 is producing today.

FSO

FSO attempts to leverage optical and laser technology advances in the fiber-optical realm to create short-range, high-bandwidth, line-of-sight, physical-layer point-to-point links through the transmission of near-infrared signals through the air. It brings the promise of multigigabit wireless transmissions but with some severe limitations that so far have precluded it from widespread acceptance and deployment. However, depending upon your specific circumstance, it just might provide a solution that can work for you, as an alternative to an 802.11 wireless bridge.

The main technological challenges that FSO links face follow:

Fog, which consists of tiny water droplets and can absorb, scatter, or reflect light, is the major challenge. Other forms of weather, such as rain and snow, have a lesser effect, although very heavy rain or blizzard conditions can also brink down a link.
Absorption, which is a function of the wavelength of the light in use, can decrease the power of the light beam. Absorption most often comes from fog or aerosols such as dust, sea salt, or man-made pollutants.
Scatter, especially when the scattering particle is similar to the wavelength, can significantly attenuate the beam intensity because it redirects energy in random directions. The scattering particles could be fog, haze, or pollutants. As the link range increases, so do the scattering losses.
Physical objects, such as birds, can actually temporarily interrupt FSO links.
Building sway can disturb the alignment of the transmitter and receiver and disrupt the link.
Turbulence, which occurs when heated objects create moving air pockets of differing temperatures, causes time-varying changes in the index of refraction at the air-pocket interfaces. It can result in beam wander as it randomly reflects through the pockets, scintillation in the form of intensity fluctuations, and increased beam spread.

Fortunately for the FSO community, the two most common fiber-optic communication wavelengths, 850 and 1550 nanometer (nm), happen to align with two atmospheric absorption windows.

A simple FSO transmitter consists of an LED or laser light source connected to a telescope formed from lenses or mirrors and a receiver that has a similar optical assembly that focuses light energy on a photo detector. The use of LEDs, while providing a cheap solution, in general limits the bandwidth to the hundreds of Mbps over much shorter ranges than lasers can achieve. Because semiconductor lasers are fairly small and high power, and because they are in use by the fiber-optic community as well, most FSO vendors build their systems around these components. The optical subsystem of mirrors and lenses usually contributes the most to the size of FSO systems and requires a very precise and costly calibration and alignment procedure that must be maintained across temperature variations as the lenses and mirrors expand and contract.

To achieve any significant ranges, it is necessary to use a very narrow beam divergence, such as a milliradiant. As you move the receiver away from transmitter, the beam diverges to diameters that are larger than the receive telescope, and any transmitted energy that is not collected results in geometrical path loss. For example, as shown in Figure 9-3, with a 2-milliradiant beam divergence and a link range of 500 m, the beam diameter will be 1 m. However, if the receiving optics collects energy from a 10 centimeter (cm) diameter region, only 1 percent of the energy will be collected, for a 20 dB loss in the link budget. For every doubling of the distance, the geometric path loss is increased by 3 dB in clear air.

Figure 9-3. Geometric Path Loss Example

Decreasing the beam divergence makes the initial alignment of the link more challenging and also makes it more susceptible to building sway, which can result in a pointing loss. To combat this problem and enable the use of narrower beams, you use tracking and acquisition systems. They usually include an auto-tracking feature that can use a deflection-detection system, such as an array of detectors. With these systems, the output is processed in real time to drive a gimbal that adjusts in the vertical and horizontal plane.

The installation process for an FSO link can be somewhat more time-consuming than that for an 802.11 wireless bridging link, mainly because of the many previously discussed challenges. You must meticulously plan the site survey with a significant fade margin built in for environmental effects. During the installation, you must take care to keep the link far enough above sources of air turbulence, and the alignment itself must be pinpoint. Because of the potential harmful effects to your eyes, you must take care with the high-power lasers, especially with equipment operating with an 850 nm wavelength because that frequency can easily penetrate the eye.

Despite all these caveats, the promise of Gbps links at a fraction of the cost of fiber trenching might make this technology viable for your application. Similar to 802.11, FSO operates in an unlicensed manner, but unlike radios, they are not subject to interference. With the proper precautions and planning, you can end up with a solution that provides years of reliable service.

100 Mbps WLANs

Several companies offer 108 Mbps WLANs today, but because there is no standard, they don't interoperate. In general, they combine two of the available 802.11a channels, forming a single "new channel" that is twice as wide as a standard channel. In the near future, we might see the formation of a higher throughput task group in 802.11. It is anticipated that such a task group would not only focus on achieving a 100 Mbps data rate, but also strive for a 100 Mbps throughput experience for usersbecause it is what they have come to expect from their wired LANs. To achieve this experience, it will need to make modifications to the 802.11 physical layer (PHY) and the 802.11 MAC. The group must weigh questions of coexistence and backward compatibility in addition to those of basic viability regarding spectral efficiency, range, and power consumption.

From a usage-profile perspective, the two main drivers for 100 Mbps throughput WLANs will be throughput equivalence with wired 100BASE-T Ethernet and wireless multimedia for the home. The former will further the cause of the fully wireless office because wireless will provide the same throughput experience as wired. The latter will be driven by the desire to provide high-quality audio and video to all parts of the home without wiring and also support Internet surfing.

Summary

This chapter considered three complementary technologies. UWB will most likely be a replacement for Bluetooth because it is seeking to address the same wireless PAN space as Bluetooth but with much higher data rates. FSO has been deployed as a point-to-point technology when the right conditions exist, but because it still has not gained mainstream acceptance, it will likely be surpassed or integrated with point-to-point radio techniques. The real future of 802.11 lies with a 100 Mbps WLAN standard that will be the next quantum step forward after 802.11g and 802.11a.

Referency Ebook: Pejman Roshan, Jonathan Leary, 802.11 Wireless LAN Fundamentals, Cisco Press, 2003, Indianapolis, Indiana 46240 USA